1. Field of Invention
This invention relates to a computer, particularly to a computer having an expanded addressing capability.
2. Description of Prior Art
Digital computers generally have a limited addressing capability, i.e. the capability to generate, write information to, and read information from, only up to a given number of memory addresses or locations. For example, in a computer operating on a binary radix (i.e. a radix of two), and having sixteen address lines, the computer can generate up to 65536 addresses on its address lines; these addresses can have any value from 0 to 65535. Each address may be a location in a memory or an input-output (I/O) port which connects the computer to addressable peripheral equipment, such as a keyboard, a printer, or a video display terminal. Each memory location usually consists of eight binary cells which can store a byte (information word) consisting of eight bits (ONES and ZEROES) of binary information, and each I/O address can pass eight-bit bytes to and from the computer.
While the capability of addressing 65536 locations, which can contain 65536.times.8=524,288 binary cells, may seem like a relatively large capacity, in actuality this amount of storage capability is considered relatively limited, even for a small computer, and severly limits the program and data handling and storage capability of the computer and the number of items of peripheral equipment which can be connected to the computer.
It is possible to increase the addressing capability of computer by providing additional address lines and making corresponding expansions in its program counter, instruction register, decoder, etc. However, it is difficult to do so since most small computers are now formed within a monolithic integrated circuit (usually termed a microprocessor) and the number of leads or output terminals of the microprocessor or "chip" are limited by package size. Also, many microprocessors have become standardized for mass production and therefore have relatively low cost, so that increasing addressing capability requires redesign and commercial implementation of new and non-standard microprocessors at extremely great expense. Therefore it would be desirable to provide a means for expanding the addressing capability of a computer without increasing the number of its address lines or making other concomitant changes therein.
Heretofore, one method of increasing the address capability has been to provide a memory bank switching capability in which, in a given address range, two different banks of memory are provided. Either bank was selectable in a separate bank-select cycle prior to the regular fetch/execute cycle. While the use of the bank selection or bank switching concept has been able to expand the addressing capability of a computer beyond its stated capacity, the degree of expansion possible was extremely limited and merely increased memory capacity slightly, such that a slightly longer program or some additional data could be stored. Most of the aforementioned limitations were still extant and the peripheral equipment capacity of the computer was not increased.
In prior bank switching or bank selection techniques, a non-selected memory bank was enabled or switched into an address range by supplying an enabling signal to latch in the memory bank prior to the normal memory read or write operation. Most banks of read-only memories (ROMs) were provided in form of a monolithic integrated circuit (IC) or chip which was packaged in a standard or custom integrated circuit package having a plurality of terminals or leads, one of which was the "chip select" or "chip enable" terminal. When this terminal was supplied with an active signal, the chip was "selected" so that it would supply output data in response to an address input.
This method of chip selection was disadvantageous in that it required a separate terminal on the IC where, as stated, the number of available terminals was limited due to size configurations. Also, the IC consumed power whether it was selected or not. While such power consumption was not excessive, any power consumption, especially for unused circuitry, is wasteful and is especially undesirable in portable devices which are operated from an energy cell where the energy supply capability is relatively limited.